1. Field of the Inventive Concept(s)
The presently disclosed and/or claimed inventive concept(s) relates generally to oxidized reaction products made from the mechanocatalytic oxidative depolymerization of lignin. More particularly, but without limitation, the mechanocatalytic oxidative depolymerization of lignin is performed in a non-aqueous and solvent-free process, i.e., via a solid-solid mechanocatalytic oxidative reaction methodology. In one particular embodiment, the process of making such oxidized reaction products includes, without limitation, the step of mechanocatalytically reacting an oxidation catalyst with lignin or a lignin-containing material. The oxidized reaction products obtained from the process include, for example, at least one of vanillin, syringealdehyde, vanillic acid, and syringic acid.
2. Background of the Inventive Concept(s)
The conversion of lignocellulosic biomass represents a potentially rich source of aromatic compounds and complete depolymerization of lignin within the lignocellulosic biomass can produce salable products such as vanillin, syringealdehyde, vanillic acid, syringic acid, and specialty chemicals that use these compounds as precursor molecules. Markets for these biomass-based materials will expand as demand grows for non-petroleum sourced materials, for example. Current production methods for the extraction of vanillin from Kraft liquor, for example, produce 160 kg of caustic waste for every kilogram of vanillin produced. Profitability can be increased and environmental concerns can be lessened by the development of a scalable process that foregoes such traditional caustic processes for the conversion of biomass materials.
Lignin is a complex chemical compound (shown in FIG. 1) commonly derived from wood as a byproduct of the pulp industry and is an integral part of the secondary cell walls of plants and some algae. It is one of the most abundant organic polymers on Earth, exceeded only by cellulose, embodying approximately 30% of non-fossil organic carbon, and constituting from a quarter to a third of the dry mass of wood. As a biopolymer, lignin is unusual because of its heterogeneity and lack of a defined primary structure. Its most commonly noted function is the support through strengthening of wood (xylem cells) in trees. Global production of lignin is around 1.1 million metric tons per year and is used in a wide range of low volume, niche applications where the form of lignin, but not its quality, is important.
Lignin is a cross-linked racemic macromolecule with molecular masses in excess of 10,000. It is relatively hydrophobic and aromatic in nature. The degree of polymerization in nature is difficult to measure since it is fragmented during extraction, and the molecule consists of various types of substructures that appear to repeat in a haphazard manner (as shown in FIG. 1). There are three monolignol monomers that are methoxylated to various degrees: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. These lignols are incorporated into lignin in the form of the phenylpropanoids p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S), respectively. Gymnosperms have a lignin content that consists almost entirely of G with small quantities of H, the lignin content of dicotyledonous angiosperms is mostly a mixture of G and S (with very little H), and monocotyledonous lignin is a mixture of all three. Many grasses have mostly G type lignin structures, while some palms have mainly S type lignin structures. The type and amount of lignin depolymerization products (e.g., oxidized reaction products) are dependent on the type and/or amount of a particular type of lignin in the biomass (i.e., H, G, and/or S). That is, the available percentage of precursors in the lignin structure strictly determines the formation of phenolic compounds such as vanillin or syringealdehyde. Lignin is especially useful in producing phenolic aldehydes as it requires fewer transformations or chemical treatments. For example, Sun et al., (2000) found that a yield of about 50 to 59.7% syringaldehyde and vanillin in equal proportions of the total phenolic aldehydes was obtained via nitrobenzene oxidation from lignin extracted from rice straw.
Biodegradation of lignin is a prerequisite for processing biofuel from plant raw materials. Lignin is indigestible by animal enzymes, but some fungi and bacteria are able to secrete ligninases (also named lignases) that are able to inefficiently and non-economically biodegrade the lignin polymer. As such, the presence of lignin within cellulosic or hemicellulosic structures is associated with reduced digestibility of the overall plant biomass.
Vanillin is a phenolic aldehyde having the molecular formula C8H8O3 (shown in FIG. 2). Its functional groups include aldehyde, ether, and phenol. It is the primary component of the extract of the vanilla bean. Synthetic vanillin, instead of natural vanilla extract, is sometimes used as a flavoring agent in foods, beverages, and pharmaceuticals. Natural “vanilla extract” is a mixture of several hundred different compounds in addition to vanillin. Artificial vanilla flavoring, on the other hand, is a solution of pure vanillin, usually of synthetic origin.
Due to the scarcity and expense of natural vanilla extract, there has long been interest in the synthetic preparation of its predominant component vanillin. The first commercial synthesis of vanillin began with the more readily available natural compound eugenol. Typically, artificial vanillin is made either from guaiacol or from lignin. Lignin-based artificial vanilla flavoring is alleged to have a richer flavor profile than oil-based flavoring; the difference is most likely due to the presence of acetovanillone in the lignin-derived product, an impurity not found in vanillin synthesized from guaiacol. Synthetic vanillin became significantly more available in the 1930s, when production began using the lignin-containing waste produced by the sulfite pulping process for preparing wood pulp for the paper industry. While some vanillin is still made from lignin wastes, most synthetic vanillin is today synthesized in a two-step process from the petrochemical precursors guaiacol and glyoxylic acid.
Several routes exist for synthesizing vanillin from guaiacol. At present, the most significant of these is a two-step process practiced by Rhodia since the 1970s, in which guaiacol reacts with glyoxylic acid by electrophilic aromatic substitution. The resulting vanillylmandelic acid is then converted via 4-Hydroxy-3-methoxyphenylglyoxylic acid to vanillin by oxidative decarboxylation.
The largest use of vanillin is as a flavoring, usually in sweet foods. The ice cream and chocolate industries together comprise 75% of the market for vanillin as a flavoring, with smaller amounts being used in confections and baked goods. Vanillin is also used in the fragrance industry, in perfumes, and to mask unpleasant odors or tastes in medicines, livestock fodder, and cleaning products. Vanillin has also been used as a chemical intermediate in the production of pharmaceuticals and other fine chemicals. In 1970, more than half the world's vanillin production was used in the synthesis of other chemicals, but as of 2004 such a use only accounted for 13% of the market for vanillin. Additionally, vanillin can be used as a general purpose stain for developing thin layer chromatography (TLC) plates to aid in visualizing components of a reaction mixture.
Vanillic acid (4-hydroxy-3-methoxybenzoic acid) is an odorless dihydroxybenzoic acid derivative and having the formula C8H8O4 that is used as a flavoring agent. It is an oxidized form of vanillin. It is also an intermediate in the production of vanillin from ferulic acid. The highest amount of vanillic acid in plants known is found in the root of Angelica sinensis, an herb indigenous to China, which is used in traditional Chinese medicine. Açaí oil, obtained from the fruit of the açaí palm (Euterpe oleracea), is rich in vanillic acid (1,616±94 mg/kg), for example. It is also one of the main natural phenols in argan oil and is also commonly found in wine and vinegar. Vanillic acid is one of the main catechins metabolites found in humans after consumption of green tea infusions.
Syringealdehyde (also, “syringaldehyde” or 3,5-dimethoxy-4-hydroxybenzaldehyde) is an organic compound having the formula C9H10O4 (shown in FIG. 3) that occurs in trace amounts throughout nature. Because it may contain many different functional groups, it can be classified in many ways—aromatic, aldehyde, or phenol. It is a colorless solid (impure samples appear yellowish) that is soluble in alcohol and polar organic solvents. Its refractive index is 1.53. Syringealdehyde is very similar in structure to vanillin and has comparable applications. Though not as well-commercialized as vanillin, syringealdehyde chemistry and its manipulation are emerging rather rapidly.
Syringealdehyde is formed in oak barrels and blends into whiskey, giving it a spicy, smoky, hot, and smoldering wood aroma. It is also used in the manufacture of antibacterial drugs including Trimethoprim, Bactrim, and Biseptol where syringealdehyde is an essential intermediate in their production. Bactrim or Biseptol are combinations of Trimethoprim with sulfamethoxazole. Applications for the use of syringealdehyde are diverse: as an antifungal agent for yeast infections and as an antimicrobial for clostridium; as an antimicrobial additive to antiseptic paper (thereby reducing the transmission of staph, pneumonia, and pseudomonas bacteria); and, it has potent antioxidant properties. See, e.g., Ibrahim et al., “A Concise Review of the Natural Existance, Synthesis, Properties, and Applications of Syringaldehyde,” BioResources 7(3) (2012), the entire contents of which is hereby incorporated by reference in its entirety. For example, antioxidant activity for syringealdehyde has been recorded to be six times higher than that of protocatechuic aldehyde and the antioxidant activity of syringealdehyde has been found to be significantly greater than that of vanillin. (Boundagidou et al., 2010).
Lignin, as it is a waste of the pulping industry and a major by-product from the biomass-to-ethanol conversion process, offers a continuous, renewable, and economical supply of syringealdehyde. Syringyl (S) units found in lignin are the source from which syringealdehyde can be obtained when lignin-containing materials undergo certain oxidation reactions.
Syringic acid (4-hydroxy-3,5-dimethoxybenzoic acid) is a naturally occurring O-methylated trihydroxybenzoic acid having a formula of C9H10O5. It is an oxidized form of syringealdehyde. Syringic acid (as well as vanillic acid) possesses antimicrobial, anti-cancer, and anti-DNA oxidation properties. Additionally, both compounds act as immunomodulators and provide protective effects in mice with liver injuries. See, e.g., Itoh et al. “Hepatoprotective Effect of Syringic Acid and Vanillic Acid on CCI4-Induced Liver Injury,” Biological and Pharmaceutical Bulletin, Vol. 33 (2010) No. 6 P 983-987, the entire contents of which is expressly incorporated by reference herein.
Mechanocatalysis or tribocatalysis is a solid-solid reaction using mechanical force without the addition of solvents, i.e., it is a non-aqueous or solvent-free catalytic reaction. Effective mechanocatalysts are mechanically robust and possess sites that are physically accessible and chemically active. Mechanocatalytic processes also typically do not require external heat. Substantially all of the energy for the reaction comes from the pressures and frictional heating provided by the kinetic energy of milling media moving in a container. In a mechanocatalytic system, it is important that intimate contact between the catalyst and reactant is maintained. Pebble (or rolling) mills, shaker mills, attrition mills, and planetary mills are a few examples of mills that effectively “push” the catalyst into contact with the material to be treated in a mechanocatalytic process. A mechanocatalytic process for converting biomass to soluble sugars is, for example, disclosed in U.S. Ser. No. 11/935,712, the entire contents of which are hereby incorporated by reference in their entirety.
One of the ways to convert lignin to fuels or chemicals is by base catalyzed depolymerization followed by hydrotreating, as shown in U.S. Patent publications 2003/0100807A1 and 2003/0115792A1. This process uses a strong base to partially break up the lignin compounds. One problem of this approach is the high consumption of strong base (such as NaOH) which makes the process less economical and environmentally appropriate. One recent study showed, for example, that pH within this process must be above 12.4 in order to achieve a relatively high lignin conversion.
Processes that avoid such a need for strong bases can shift the recovery of organic precursors from biomass to economically viable processes, as well as lessen the cost of environmental protection for such bio-conversion processes.
As such, disclosed and/or claimed herein are processes and methods for economically, safely, and reliably producing oxidized reaction products made from the mechanocatalytic oxidative depolymerization of lignin. More particularly, but without limitation, the mechanocatalytic oxidative depolymerization of lignin is performed in a non-aqueous/solvent-free based process, i.e., via a solid-solid mechanocatalytic oxidative reaction methodology. In one particular embodiment, the process of making such oxidized reaction products includes, without limitation, the step of mechanocatalytically reacting an oxidation catalyst with lignin or a lignin-containing material. The oxidized reaction products obtained from the process include, for example, at least one of vanillin, syringealdehyde, vanillic acid, and syringic acid.